The United States announced that Afghanistan and Myanmar, the world's top opium and heroin producers, would face continued US sanctions under the annual US review of nations engaged in the illicit drug trade.

Twenty out of 24 nations which the United States considers major producers or conduits of illicit drugs were certified in the review Thursday as fully cooperating with the United States.

Four nations were decertified, but sanctions against Cambodia and Haiti, which included an end to most US aid and a block on borrowing from multilateral organisations, were waived.

For the second year in a row, Afghanistan and Myanmar were the sole countries decertified and under sanctions following the release of the list issued by the White House following recommendations from the State Department.

In its annual report on drug producing countries, issued along with the certification list, the State Department said Afghanistan and Myanmar were number one and two respectively in opium and heroin production.

"Afghanistan continues to be the world's largest opium producer after another year of major increases. Afghanistan now accounts for 72 percent of the world's illicit opium supply," said the report.

"The Taliban issued in late July a new ban on poppy cultivation.

However, it is not clear how serious the Taliban's efforts are to enforce the ban."

As for Myanmar, despite efforts by the ruling junta at crop eradication and bad weather that reduced harvest, the country "remains the world's second largest producer of illicit opium and heroin."

Production of coca -- the raw ingredient of cocaine -- increased in Colombia to 583,000 tonnes compared with 521,400, while the world increase in coca production rose six percent.

The other two top coca producers, South American nations Peru and Bolivia, both reduced production, however.

Sanctions were not imposed on the three Andean nations because

of their anti-drug policy, deemed satisfactory by Washington which has pledged 1.3 billion dollars to the region, most to be used for Colombia's anti-drug initiative Plan Colombia.

The list of countries up for US certification has dropped to 24 from 26 last year. Hong Kong and Taiwan are no longer considered conduits for illegal drugs.

Nigeria and Paraguay, both decertified with waivers last year, were certified along with Mexico, China, Colombia and Pakistan.

Rand Beers, assistant secretary of state for narcotics, told a Senate Foreign Relations Committee hearing Cambodia and Haiti were granted waivers out of national interest.